The invention relates to indium phosphide (InP) semiconductor devices, and more particularly to a method of improving the efficiency of InP solar cells and other photo-receivers, and the improved cells produced thereby. InP solar cells have excellent radiation tolerance and the potential for extremely high efficiency. However, to date InP solar cells have not equalled the efficiency of other materials such as GaAs because of its propensity for high recombination of minority carriers at the front surface of the cell. The purpose of this invention is to increase the efficiency of InP solar cells by reducing the recombination of minority carriers at the front surface.
In InP, one of the most significant causes of poor efficiency is the recombination of electrons and holes at the front surface of the device, a process called surface recombination. Experimentally measured values of surface recombination velocity (SRV) on InP solar cells are extremely high. The surface recombination velocity calculated from measured values of the short wavelength quantum efficiency and current-voltage characteristics of the best existing InP solar cells is in the range of 5.times.10.sup.6 to 10.sup.7 cm/s, which is unfavorably high. Once recombined at the surface, the carriers are lost and cannot contribute to either the output voltage or the output current of the cell.
High front surface recombination in InP cells is typically addressed by using n-on-p cell configurations, where n-type InP is used as the front surface or "emitter" layer. High doping (n.sup.+) levels in the n-InP and extremely shallow emitter layers contribute to minimize surface losses in the emitter. However, the best n-on-p type InP cells have 19% conversion efficiency measured at Air Mass Zero (AM0), and the best p-on-n InP cells today have efficiencies of 15% AM0. Thus, with the present technology, p-on-n cells are not as efficient as n-on-p cells, and neither is as efficient as GaAs cells, which have been manufactured with over 22% efficiency.
In front surface recombination velocity could be reduced, theoretical studies by the inventors indicate that p-on-n cells would have better open circuit voltage and higher efficiency than n-on-p cells. The heavy doping of the emitter required in n-on-p cells to minimize the adverse effects of high surface recombination leads to efficiency loss. The shallow emitter required is difficult to fabricate and has high resistance, leading to further losses.
Some InP solar cells have been made using as the active material InP which has been deposited or "grown" on a substrate of a different crystalline material. This is advantageous because a substrate may be used which is stronger, lighter or lower in cost than single crystal wafers of InP, while retaining all of the features of an InP solar cell. Two crystalline substrates of particular interest are silicon and germanium, which are available in wafer form. A difficulty in this approach, however, is that in the process of growing the InP, the Si or Ge will tend to diffuse from the substrate into the InP layer being grown, doping the InP to n-type. The p-on-n type solar cell is preferable for this application because it can make use of the n-type doping in the base. Thus, reduction of surface recombination in p-on-n type InP is particularly important for these cell designs.
InP solar cells can also be used as one element in a tandem solar cell, where it is used in conjunction with other solar cell materials. For example, an InP solar cell may be used on top of an InGaAsP solar cell, so that the light which penetrates through the InP can be usefully absorbed by the InGaAsP.
Finally, while the devices discussed in detail are solar cells, other electronic devices designed to absorb light, such as photodetectors, photodiodes, phototransistors, laser power receivers, thermophotovoltaic cells and the like could also be fabricated out of InP and would likewise benefit from reduced surface recombination at the light absorbing surface. Hence, there is a need for alternative methods of minimizing the effects of high surface recombination velocities in InP that do not have the disadvantages associated with current methods. By increasing the efficiency of InP devices, and in particular p-on-n InP, in this way, they will become more suitable for space power applications where their excellent radiation tolerance can be exploited.